In certain circumstances, it becomes necessary to closely position multiple omni directional antennas, such as those used in repeaters, where the antennas for both the donor and subscriber sides of the repeater are placed in close proximity. For example, such closely spaced antennas can be embedded onto low cost printed circuit boards for use in various communications products and systems, such as in the WiDeFi™ TDD based repeater system. It is further desirable for such closely spaced antennas to maintain minimal antenna-to-antenna interaction while maintaining good gain characteristics, to be easily producible in high volume manufacturing using low cost packaging, and to be easy for a user to operate. Further, when the antenna is placed near a reflecting surface, such as a wall, that would otherwise change the free space isolation of the antennas, a mechanism is required to reduce or cancel the effect of the interaction.
Three key problems present themselves when attempting to achieve high isolation between multiple, closely-spaced antennas that are printed on a small PCB board with near omni-directional antenna patterns and that must work in close proximity to unknown structures such as walls, furniture, and the like. The problems are coupling of radiated energy, common mode coupling and multi-path or random coupling of in-band signal energy.
In dealing with the first problem of coupling of radiated energy from one antenna into the receiver section of another, the radiated fields emanating from the antenna structure must be cancelled somehow to increase isolation. The closer the antennas are in physical proximity, the more they will tend to couple energy, which coupling reduces isolation between the antennas. Additional problems can arise when attempting to maintain an omni or semi-omni directional antenna pattern.
Dealing with the second problem of common mode coupling involves a coupling mechanism that is difficult to cancel. Common mode coupling occurs due to a shared ground on a printed circuit board. Voltage perturbations on the ground plane associated with generating and transmitting a signal from one antenna circuit couple into an adjacent antenna circuit either electrically into input circuits through the ground plane or indirectly from energy induced into the ground plane or input circuits by the transmitted signal. The problem of common mode coupling is especially difficult when multiple antennas are integrated together on a very small ground plane.
The third problem of random coupling is often the most difficult coupling mechanism to address. With random coupling, energy from indeterminate reflections or interactions with objects that change the radiation patterns or sources of localized coupling are primarily the result of antenna placement. However, attempting to determine an exact antenna placement that reduces or removes the unwanted components while preserving the desired components and the directionality is not generally successful.